In liquid chromatography, which is used to analyze small fluid samples, it has become common practice to use a sample injection device which receives a liquid sample at substantially atmospheric pressure and stores it in a sample loop formed by a small diameter but long length tube. Upon subsequent actuation of the injector device, the sample is swept at a high pressure, typically 100 to 10,000 psi from the sample loop to a chromatographic column, by a solvent received from a high pressure pump. It is important to minimize mixing of the sample with solvent, since such mixing can cause the leading edge of the sample entering the chromatographic column to represent a mixture of the sample to be analyzed and the solvent, such mixing often being referred to as dispersion.
It is common practice to provide a tube coupling hole in the frame of the injector and column devices, which receive ends of the small diameter tube in an arrangement that minimizes dispersion. U.S. Pat. No. 4,182,184 shows such a tube coupling hole. The inner end of the tube coupling hole is cylindrical and of about the same diameter as the tube to closely receive it; this minimizes the "dead" space around the end portion of the tube up to the seal, which may contain solvent that will mix with the sample. The most common tube size, which may be considered to be the "standard" tube size, is a tube of 1/16th inch outside diameter. Thus, most injector and column frames have tube-receiving holes whose inner ends are cylindrical and of a diameter of about 1/16th inch.
The size of the cavity between the extreme tip of the tube and a shoulder at the inner end of the tube coupling hole, varies in size, and can be substantial if the tip of the tube is not cut perfectly square at the end, if the loop is not pushed completely into the tube coupling hole, or if there is a burr or particle trapped between the tip of the tube and the inner end of the hole. The increased dispersion resulting from a larger cavity, is generally not important where a medium to larger sample is stored and is analyzed by a medium to larger column. However, in some situations, the sample volume is no more than about 2 uL (microliters) and is analyzed by a column of about 1 to 2 mm (millimeters) inside diameter. The dispersion caused by a larger than usual cavity at the end of a standard size tube can be significant. For example, a one-hundredth inch length between the tip of a standard size tube and the end of the tube receiving hole, can increase the dispersion by 22 uL.sup.2 (microliters squared) if the sample is injected at a rate of 200 uL/min. It is very difficult to see, by the unaided eye, that the ferrule which holds the tube is put on with the tubing too short by only one-hundredth inch.
It is possible to minimize the "dispersion volume" at the end of a tube, by using a much smaller diameter tube when small volumes of a sample are to be analyzed. For example, a tube having an outside diameter of 0.020 inch can be used instead of a standard tube of 0.0625 inch, with the cross sectional area of the outside of the smaller tube being about one tenth that of the larger tube. The length of any gap at the end of the smaller tube is likely to be smaller than for the larger tube, but even for the same length of gap the volume of solvent trapped in the gap will only be about one tenth as much. Such a small diameter tube is difficult to use for larger fluid samples, since it may require very high pressures to rapidly pump out the sample from the tube and a much longer length of tube is required to hold the sample. An injector or column frame which was constructed to readily receive a standard size tube and also a much smaller diameter tube, and effective to seal to either one of them, with a minimum dispersion gap for the smaller size tube, all in a sample and low cost arrangement that enables easy switching between tube sizes, would be of considerable value.